Action of brassinosteroids in the cotton leafworm Spodoptera littoralis

A gustatory receptor tuned to the steroid plant hormone brassinolide in Plutella xylostella (Lepidoptera: Plutellidae)

Feeding and oviposition deterrents help phytophagous insects to identify host plants. The taste organs of phytophagous insects contain bitter gustatory receptors (GRs). To explore their function, the GRs in Plutella xylostella were analyzed. Through RNA sequencing and qPCR, we detected abundant PxylGr34 transcripts in the larval head and adult antennae. Functional analyses using the Xenopus oocyte expression system and 24 diverse phytochemicals showed that PxylGr34 is tuned to the canonical plant hormones brassinolide (BL) and 24-epibrassinolide (EBL). Electrophysiological analyses revealed that the medial sensilla styloconica of 4^th instar larvae are responsive to BL and EBL. Dual-choice bioassays demonstrated that BL inhibits larval feeding and female oviposition. Knock-down of PxylGr34 by RNAi attenuates the taste responses to BL, and abolishes BL-induced feeding inhibition. These results increase our understanding of how herbivorous insects detect compounds that deter feeding and oviposition, and may be useful for designing plant hormone-based pest management strategies.

Plant-eating insects use their sense of taste to decide where to feed and where to lay their eggs. They do this using taste sensors called gustatory receptors which reside in the antennae and legs of adults, and in the mouthparts of larvae. Some of these sensors detect sugars which signal to the insect that the plant is a nutritious source of food. While others detect bitter compounds, such as poisons released by plants in self-defense.

One of the most widespread plant-eating insects is the diamondback moth, which feeds and lays its eggs on cruciferous vegetable crops, like cabbage, oilseed rape and broccoli. Before laying its eggs, female diamondback moths pat the vegetable’s leaves with their antennae, tasting for the presence of chemicals. But little was known about the identity of these chemicals.

Cabbages produce large amounts of a hormone called brassinolide, which is known to play a role in plant growth. To find out whether diamondback moths can taste this hormone, Yang et al. examined all their known gustatory receptors. This revealed that the adult antennae and larval mouthparts of these moths make high levels of a receptor called PxylGr34.

To investigate the role of PxylGr34, Yang et al. genetically modified frog eggs to produce this receptor. Various tests on these receptors, as well as receptors in the mouthparts of diamondback larvae, showed that PxylGr34 is able to sense the hormone brassinolide. To find out how this affects the behavior of the moths, Yang et al. investigated how adults and larvae responded to different levels of the hormone. This revealed that the presence of brassinolide significantly decreased both larval feeding and the amount of eggs laid by adult moths.

Farmers already use brassinolide to enhance plant growth and protect crops from stress. These results suggest that the hormone might also help to shield plants from insect damage. However, more research is needed to understand how this hormone acts as a deterrent. Further studies could improve understanding of insect behavior and potentially identify more chemicals that can be used for pest control.

Therapeutic Potential of Brassinosteroids in Biomedical and Clinical Research

Steroids are a pivotal class of hormones with a key role in growth modulation and signal transduction in multicellular organisms. Synthetic steroids are widely used to cure large array of viral, fungal, bacterial, and cancerous infections. Brassinosteroids (BRs) are a natural collection of phytosterols, which have structural similarity with animal steroids. BRs are dispersed universally throughout the plant kingdom. These plant steroids are well known to modulate a plethora of physiological responses in plants leading to improvement in quality as well as yield of food crops. Moreover, they have been found to play imperative role in stress-fortification against various stresses in plants. Over a decade, BRs have conquered worldwide interest due to their diverse biological activities in animal systems. Recent studies have indicated anticancerous, antiangiogenic, antiviral, antigenotoxic, antifungal, and antibacterial bioactivities of BRs in the animal test systems. BRs inhibit replication of viruses and induce cytotoxic effects on cancerous cell lines. Keeping in view the biological activities of BRs, this review is an attempt to update the information about prospects of BRs in biomedical and clinical application.

Behavioral and metabolic effects of sublethal doses of two insecticides, chlorpyrifos and methomyl, in the Egyptian cotton leafworm, Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae)

Insecticides have long been used as the main method in limiting agricultural pests, but their widespread use has resulted in environmental pollution, development of resistances, and biodiversity reduction. The effects of insecticides at low residual doses on both the targeted crop pest species and beneficial insects have become a major concern. In particular, these low doses can induce unexpected positive (hormetic) effects on pest insects, such as surges in population growth exceeding what would have been observed without pesticide application. Methomyl and chlorpyrifos are two insecticides commonly used to control the population levels of the cotton leafworm Spodoptera littoralis, a major pest moth. The aim of the present study was to examine the effects of sublethal doses of these two pesticides, known to present a residual activity and persistence in the environment, on the moth physiology. Using a metabolomic approach, we showed that sublethal doses of methomyl and chlorpyrifos have a systemic effect on the treated insects. We also demonstrated a behavioral disruption of S. littoralis larvae exposed to sublethal doses of methomyl, whereas no effects were observed for the same doses of chlorpyrifos. Interestingly, we highlighted that sublethal doses of both pesticides did not induce a change in acetylcholinesterase activity in head of exposed larvae.

Steroid plant hormones: effects outside plant kingdom

Brassinosteroids (BS) are the first group of steroid-hormonal compounds isolated from and acting in plants. Among numerous physiological effects of BS growth stimulation and adaptogenic activities are especially remarkable. In this review, we provide evidence that BS possess similar types of activity also beyond plant kingdom at concentrations comparable with those for plants. This finding allows looking at steroids from a new point of view: how common are the mechanisms of steroid bioregulation in different types of organisms from protozoa to higher animals.

Study on ecdysteroid levels and gene expression of enzymes related to ecdysteroid biosynthesis in the larval testis of Spodoptera littoralis

We investigated here the ecdysteroid titers and the expression of six genes coding for known enzymes of the ecdysteroid biosynthesis in the testes of last instar larvae of the pest cotton leafworm, Spodoptera littoralis. We showed that the timing of the ecdysteroid profile was the same in testes and in hemolymph, with a small peak at day 2 and a large one at day 4 after ecdysis. Ecdysone and 20-hydroxyecdysone (20E) were detected in both tissues. 20E was the major ecdysteroid in testes and in hemolymph from day 4. Interestingly, the gene expression of the steroidogenetic enzymes, Neverland, and the five cytochrome P450 enzymes encoded by the Halloween genes was confirmed in the testes, and varied during the instar. However, from the data obtained so far, we cannot conclude that the measured ecdysteroids in the testes result from the activity of the genes under study. Indeed, it is suggested that the ecdysone produced centrally in the prothoracic glands, could have been transformed into 20E in the testes, where Sl-shade is well expressed.

Relationship between larval-pupal metamorphosis and transcript expression of insulin-like peptide and insulin receptor in Spodoptera littoralis

Insulin-like peptides (ILPs) affect a wide variety of biological events, such as metabolism, lifespan, growth and reproduction. Two ILPs (Spoli-ILP1 and Spoli-ILP2) were identified in the cotton leafworm, Spodoptera littoralis, while the functions and developmental characters are not fully understood. In the present study, we identified the partial sequence of a putative S. littoralis insulin receptor (Spoli-InR) and investigated the stage (age)- and tissue-dependent expression profile of Spoli-InR in addition to Spoli-ILPs during the last larval development and larval-pupal metamorphosis. Spoli-ILP1 and Spoli-ILP2 were specifically expressed in the brain, and their gene expressions were gradually decreased in concert with larval-pupal development. On the other hand, Spoli-InR was expressed in all the selected tissues (brain, testis, fat body, Malpighian tubules, prothoracic glands and midgut), though the gene expression pattern was different among the tissues. Interestingly, the transcript expression pattern of Spoli-InR in the fat body seemed to relate with larval-pupal development. In a parallel experiment, the juvenile hormone mimetic methoprene was able to prolong the larval period when applied before the commitment peak of ecdysteroids titer in the hemolymph, and in this case the expression of Spoli-ILPs and Spoli-InR was affected. These results demonstrated first a relationship between transcript expression of Spoli-ILPs and larval-pupal development, and second they suggested the effect of ILPs may be controlled by not only Spoli-ILPs expression but also Spoli-InR expression.

In vitro activity of pacifastin-like inhibitors in relation to their structural characteristics

Information on the structural characteristics and inhibitory activity of the pacifastin family is restricted to a handful of locust pacifastin-related inhibitors. In this report the optimization of a bacterial recombinant expression system is described, resulting in the high yield production of pacifastin-like inhibitors of the desert locust. Subsequently, the relative inhibitory activity of these peptides towards mammalian, locust and caterpillar digestive peptidases has been compared. In general, the enzyme specificity of locust pacifastin-like inhibitors towards trypsin- or chymotrypsin-like peptidases corresponds to the nature of the P1-residue at the reactive site. In addition, other structural characteristics, including specific core interactions, have been reported to result in a different affinity of pacifastin members towards digestive trypsin-like enzymes from mammals and arthropods. One remarkable observation in this study is a specifically designed pacifastin-like peptidase inhibitor, which, unlike other inhibitors of the same family, does not display this specificity and selectivity towards digestive enzymes from different animals.

Prospects of brassinosteroids in medicinal applications

Steroids are an imperative group of hormones which play a key role in the transmission of signals that mediate growth and physiological responses in most pluricellular organisms. Brassinosteroids (BRs), a class of plant-specific steroid hormones, control many of the developmental and physiological processes like their animal counterparts, including regulation of gene expression, cell division and expansion, differentiation, programmed cell death, and homeostasis. Recent studies have indicated that these hormones have antiviral, antifungal, antiproliferative, antibacterial, neuroprotective and immunomodulatory properties in animal system. BRs analogues have been reported to have antiviral activity against herpes simplex virus type 1 (HSV-1), arenaviruses as well as against replication of vesicular stomatitis virus (VSV) in Vero cells. Also, antiherpetic activities both in a human conjunctive cell line (IOBA-NHC) and murine herpetic stromal keratitis (HSK) experimental models have been reported. In human cells, anticancer structure-activity relationship of natural BRs revealed their high cytotoxic activity. Since, BRs and their analogues are reported to inhibit cell growth in cancer cell lines, they may be considered as promising phytohormones for potential anticancer drugs. The use of pollens in folk medicine also indicates scope of steroids of plant pollens in medicines. An attempt has been made in this paper to document the information available on the prospects of BRs in therapeutics.

Identification and expression profile of Halloween genes involved in ecdysteroid biosynthesis in Spodoptera littoralis

20-Hydroxyecdyone (20E), an active form of ecdysteroid, is the key hormone in insect growth and development. The biosynthesis of ecdysteroid is triggered and under the control of the neuropeptide, prothoracicotropic hormone (PTTH). To date, five cytochrome P450 enzymes, namely Spook (Spo), Phantom (Phm), Disembodied (Dib), Shadow (Sad) and Shade (Shd) related to ecdysteroid biosynthesis, are identified and the character of last four enzymes is well studied in Drosophila melanogaster, Bombyx mori and Manduca sexta. These genes are called Halloween genes and mediate the biosynthesis of 20E from cholesterol. In this study, we extended these works to a major pest insect in agriculture, the cotton leafworm Spodoptera littoralis (Lepidoptera: Noctuidae). We identified the sequence of five Halloween genes, and the converted amino acid sequences were compared with those of other insects. The phylogenetic analysis clearly showed separated clusters of each gene and the evolutional conservation in insects with a high similarity in Lepidoptera. Spo, phm, dib and sad were predominantly expressed in prothoracic glands, and shd was expressed in fat body and Malpighian tubules at the last instar larvae. Spo expression was kept high level between day 2 and day 4 after ecdysis. The expression of phm and dib peaked at day 2, and sad and shd expressions peaked at day 2 and day 4 after ecdysis. In addition, the hemolymph ecdysteroid titer showed a small peak at day 2 and a large peak at day 4 after ecdysis. These results suggest the importance of Halloween genes in ecdysone biosynthesis by prothoracic glands and conversion of ecdysone into 20E by fat body in larval-pupal metamorphosis.

Effect of ace inhibitors and TMOF on growth, development, and trypsin activity of larval Spodoptera littoralis

Angiotensin converting enzyme (ACE) is a zinc metallopeptidase capable of cleaving dipeptide or dipeptideamide moieties at the C-terminal end of peptides. ACE is present in the hemolymph and reproductive tissues of insects. The presence of ACE in the hemolymph and its broad substrate specificity suggests an important role in processing of bioactive peptides. This study reports the effects of ACE inhibitors on larval growth in the cotton leafworm Spodoptera littoralis. Feeding ACE inhibitors ad lib decreased the growth rate, inhibited ACE activity in the larval hemolymph, and down-regulated trypsin activity in the larval gut. These results indicate that S. littoralis ACE may influence trypsin biosynthesis in the larval gut by interacting with a trypsin-modulating oostatic factor (TMOF). Injecting third instar larvae with a combination of Aea-TMOF and the ACE inhibitor captopril, down-regulated trypsin biosynthesis in the larval gut indicating that an Aea-TMOF gut receptor analogue could be present. Injecting captopril and enalapril into newly molted fifth instar larvae stopped larval feeding and decreased weight gain. Together, these results indicate that ACE inhibitors are efficacious in stunting larval growth and ACE plays an important role in larval growth and development.

Angiotensin-converting enzyme in Spodoptera littoralis: molecular characterization, expression and activity profile during development

The characterization of the full-length angiotensin-converting enzyme (ACE) cDNA sequence of the lepidopteran Spodoptera littoralis is reported in this study. The predicted open reading frame encodes a 647 amino acids long protein (SlACE) and shows 63.6% identity with the Bombyx mori ACE sequence. A 3D-model, consisting of 26 alpha-helices and three beta-sheets, was predicted for the sequence. SlACE expression was studied in the embryonic, larval and pupal stages of S. littoralis and in different tissues of the last larval stage by reverse-transcribed PCR. This revealed that the gene is expressed throughout the life cycle and especially in brain, gut and fat body tissue of the last stage. These results are in agreement with a role of ACE in the metabolism of neuropeptides and gut hormones. In addition, ACE activity has been studied in more detail during development, making use of a fluorescent assay. High ACE peptidase activity coincides with every transition state, from embryo to larva, from larva to larva and from larva to pupa. A peak value in activity occurs during the early pupal stage. These results indicate the importance of SlACE during metamorphosis and reveal the high correlation of ACE activity with the insect's development, which is regulated by growth and developmental hormones.

Presence of angiotensin converting enzyme isoforms in larval lepidoptera (Spodoptera littoralis)

In this research the presence of angiotensin converting enzyme (ACE) in larvae of the lepidopteran Spodoptera littoralis was evaluated. Making use of the substrate Abz-FRK-(Dnp)P-OH and the specific inhibitor captopril at 10 microM, ACE activity was determined in a fluorescence assay for intact larvae, hemolymph, head, midgut and dorsal tissue. In dorsal tissue and hemolymph, ACE activity was highest. These data are consistent with a possible role for ACE in contractions of the dorsal vessel and metabolism of circulating peptide hormones in the hemolymph. After the presence of ACE was confirmed, a sequential procedure of anion exchange and size exclusion chromatography was applied to purify ACE from whole wandering larvae (last stage). With this procedure, three different ACE pools were collected that cleaved the fluorogenic substrate Abz-FRK-(Dnp)P-OH. Activity could be inhibited by a final concentration of 2.5 microM captopril. In addition, two out of three samples eluted at different salt concentration and thus ACE 1, 2 and 3 represent at least two different ACE isoforms. These data reveal that ACE is present in S. littoralis and that at least two out of three isolated ACE forms are truly isoforms.

Action of 24-epibrassinolide on a cell line of the beet armyworm, Spodoptera exigua

The Spodoptera exigua cell line Se4 is sensitive for ecdysteroid activity stimulated by the insect molting hormone, 20-hydroxyecdysone (20E), showing a cease in cell proliferation (with 50% inhibition around 1 microM) and characteristic cell morphology changes with aggregation and formation of long filamentous cytoplasmic extensions. The bisacylhydrazine tebufenozide also triggered such typical cellular effects in Se4, and in addition, it showed an affinity for binding in competition with 3H-ponasterone A (PoA) that was similar to 20E (with 50% competition around 1 microM), confirming that such non-ecdysteroids display an ecdysteroid agonist activity. In contrast, when Se4 cells were incubated with the native plant hormone 24-epibrassinolide (24BR), none of the effects triggered by 20E were observed. Hence, a competition binding experiment with 3H-PoA demonstrated no affinity of 24BR for binding to the ecdysteroid receptor in the Se4 cell line. In another series of experiments, the Se4 cell line was tested in sensitivity response to increased acetylcholinesterase (AchE) activity after treatment with ecdysteroid active compounds. The AchE activity measured in the cell line is discussed in relation to inhibition by eserine. The obtained results suggest that 24BR exerted no ecdysteroid activity.

Nonsteroidal ecdysone agonists

Nonsteroidal ecdysone agonists are novel compounds that have become attractive candidates not only as pest control agents in agriculture but also as tools for research. Their narrow spectrum of activity makes them relatively safe as pesticides, and their mode of action as ligands for gene expression has found application in gene therapy and inducing transgenic gene expression in plants. These diacylhydrazines (DAHs) are potent nonsteroidal ecdysone agonists, and four of them, tebufenozide, methoxyfenozide, chromafenozide, and halofenozide, have been developed as insecticides. Although these compounds are very toxic to insects, they are safe for mammals and are environmentally benign. Their action on insects is also selective, the first three are effective against Lepidoptera but weakly active or inactive on Diptera and Coleoptera. On the other hand, halofenozide is effective on Coleoptera but mildly active on Lepidoptera. Previous reviews on ecdysone agonists have concentrated on the biological response of some DAHs and their effects on pests. In this review, the chemistry, biological effects and their modes of action at the molecular level will be covered. In addition, a few studies on other nonsteroidal ecdysone agonists, such as 3,5-di-tert-butyl-4-hydroxy-N-iso-butylbenzamide, acylaminoketones, and benzoyl-1,2,3,4-tetrahydroquinolines, will be briefly reviewed.

The angiotensin converting enzyme inhibitor captopril reduces oviposition and ecdysteroid levels in Lepidoptera

The role of angiotensin converting enzyme (ACE, peptidyl dipeptidase A) in metamorphic- and reproductive-related events in the Egyptian cotton leafworm, Spodoptera littoralis (Lepidoptera, Noctuidae) was studied by using the selective ACE inhibitor captopril. Although oral administration of captopril had no effect on larval growth, topical administration to new pupae resulted in a large decrease of successful adult formation. Oviposition and overall appearance of adults emerging from treated larvae did not differ significantly from those emerging from non-treated larvae. In contrast, topical or oral administration of captopril to newly emerged adults caused a reduction in oviposition. By evaluating the effect of captopril on ecdysteroid titers and trypsin activity, we revealed an additional physiological role for ACE. Captopril exerted an inhibitory effect on ecdysteroid levels in female but not in male adults. Larvae fed a diet containing captopril exhibited increased trypsin activity. A similar captopril-induced increase in trypsin activity was observed in female adults. In male adults, however, captopril elicited reduced levels of trypsin activity. Our results suggest that captopril downregulates oviposition by two independent pathways, one through ecdysteroid biosynthesis regulation, and the other through regulation of trypsin activity. Apparently, fecundity is influenced by a complex interaction of ACE, trypsin activity, and ecdysteroid levels.

Stereoselective synthesis of (22R)- and (22S)-castasterone/ponasterone A hybrid compounds and evaluation of their molting hormone activity

Two stereoisomers of a castasterone/ponasterone A hybrid compound, the (20R,22R) and (20R,22S)-isomers of 2alpha,3alpha,20,22-tetrahydroxy-5alpha-cholestan-6-one, were synthesized stereoselectively and their binding activity to the ecdysteroid receptor was determined. From the concentration-response curve for the inhibition of the incorporation of tritiated ponasterone A into ecdysteroid receptor containing insect cells, the concentration (IC50) required to inhibit 50% of the incorporation of radioactivity into cells was evaluated. The IC50 values of the (22R)- and (22S)-isomers were determined to be 0.30 and 38.9 microM against Kc cells, respectively, indicating that the (22R)-isomer is about 100 times more potent than the corresponding (22S)-isomer. IC50 values of these compounds against lepidopteran Sf-9 cells were determined to be 0.36 and 12.9 microM, respectively. The molting hormonal effect was examined in a Chilo suppressalis integument system and the 50% effective concentration for the stimulation of N-acetylglucosamine incorporation into the cultured integument was determined to be 2.7 microM for the (22R)-isomer, while the (22S)-isomer was inactive. On the other hand, both isomers did not show brassinolide-like activity in the rice lamina inclination assay.